1. TECHNICAL FIELD
This invention relates generally to fumigation processes for structural fumigation utilizing substantially reduced concentrations of toxic agent so as to be less hazardous during application and result in substantially less hazardous quantities of residue at the completion of the process, and more particularly, to the use of carbon dioxide as an agent to carry, disperse and increase the effect of a toxic agent such as methyl bromide, to be used to eradicate pests located in the structure to be fumigated.
2. BACKGROUND ART
Conventional structural fumigation processes have required high concentrations of a toxic agent in order to achieve effective levels of eradication of a target pest infesting the structure. If the toxic agent is methyl bromide, the present practice of using at least three pounds of the agent per one thousand cubic feet of volume in order to achieve an effective concentration of the methyl bromide. However, by reason of currently effective United States statutes and treaties, the quantity of methyl bromide used in fumigants must be reduced by one-half by the year 2000. The conventional structural fumigation processes by the use of methyl bromide alone do not provide for satisfactory kill rates for the target pests at the reduced concentrations to be required for environmental reasons. In this respect, tests by environmental agencies have found that use of methyl bromide at such conventional concentrations has the potential to result in toxic residues within protected areas which remain at a level above five parts per million in air samples after the process is completed. This level of residual toxic agent constitutes an unacceptable risk to the persons applying the fumigant as well as residents of structures and workers entering the fumigated area after the process has been completed.
Toxic agents other than methyl bromide have been used in fumigation processes. The levels of certain other toxic agents required to achieve the similar toxicity effects also often result in currently unacceptable levels of toxic residue, so as to require long aeration periods after the process has been completed in order to disperse the fumigant into the atmosphere.
Typical of the attempts to solve the problem of residual toxicity in commodities being fumigated have attempted to use carbon dioxide to control insects in stored grain. This method, set forth in U.S. Department of Agriculture bulletin AAT-S-13/April 1980, requires a carbon dioxide concentration of about 60% to achieve 95% control of most stored grain insects after a four day exposure at temperatures of twenty-seven degrees Celsius or higher. An alternate method of using a low oxygennitrogen atmosphere must be held for ten or more days. Neither of these methods is acceptable for structural fumigation.
U.S. Pat. No. 4,989,363 discloses a process for protecting stored commodities in substantially a gasfilled enclosure of a pesticidal atmosphere comprising 60-100% carbon dioxide, which is impractical for structural fumigation.
U. S. Pat. Nos. 4,651,263 and 4,756,117 have attempted to solve the problem. They teach that bulk commodities such as grain infested by pests may be fumigated with phosphine gas. However, at low concentrations, the gas must be maintained three to four weeks, which is unacceptable for structural fumigation. The time may be shortened if the atmosphere is enriched with up to 30% carbon dioxide. However, the temperature of this process is critical as phosphine gas is explosive. Therefore, raising the temperature of the fumigant gas to decrease the concentration of the fumigant gas by weight is not practical for safety reasons in structural fumigation.
Thus there has long been a need for a structural fumigation process which may utilize toxic agents at lower levels of concentration than presently used in the art while providing a completed fumigation process in a shorter time than has been possible heretofore.
It is desired that at the end of this process the residual levels of toxic agent be significantly reduced to lower, safer toxic limits so that the structure treated may be aerated and available for use earlier than heretofore practical without unacceptable residual levels of toxic agents which would otherwise result in an unacceptable health hazard.
It is further desired that the process itself be environmentally safer than those process presently used in the art. It is further desired that the gasses used be non-volatile so that they may be heated to allow the gasses to expand to fill the volume being treated thereby reducing the weight of fumigant acquired by the process to be ultimately released into the atmosphere.